1. Field of the Invention
The present invention relates to an ionization-type smoke sensor having a 2-chamber 1-radiation source structure which includes a radiation source employed to sense a change in the ion current caused by a change in the smoke concentration, and thereby detect a fire.
2. Related Art
For use in such an ionization-type smoke sensor where a 2-chamber 1-radiation source structure is adopted, the present inventors have previously proposed a structure of the electrode section of an ionization-type smoke sensor designed to be small and thin (Japanese Utility Model Registration Application No. 198631/1987). This sensor is shown in FIG. 9.
The ionization-type smoke sensor shown in FIG. 9 includes an inner electrode having a radiation source, an intermediate electrode having an opening through which radiation passes, and an outer electrode through which smoke can flow from the outside to the inside of the sensor. In the sensor, the inner electrode and the intermediate electrode define therebetween an inner chamber serving as a reference chamber where the interelectrode voltage is free from the influence of inflowing smoke and is always kept at a substantially constant level. The intermediate electrode and the outer electrode define therebetween an outer chamber where the interelectrode voltage changes in accordance with a change in the smoke concentration caused by an inflow of smoke from outside. An element having a high input impedance, such as an FET, is used to detect a change in the interelectrode voltage in the outer chamber upon an inflow of smoke, and, on the basis of the detection, the existence of smoke is determined. More specifically, as shown in FIG. 9, an inner electrode 1 is electrically connected, at an upper portion thereof, with a printed circuit board, not shown. A radiation source is disposed at a lower portion of the electrode 1. An intermediate electrode 2 is positioned axially outward (downward, as viewed in FIG. 9) of the inner electrode 1, with an inner chamber (inner ionization chamber) A being defined between the intermediate electrode 2 and the inner electrode 1. As specifically shown in FIG. 10, the intermediate electrode 2 is doughnut-shaped, and has an opening 3 through which radiation from the radiation source provided on the inner electrode 1 passes. The intermediate electrode 2 also has a single electrode member 4 extending toward the center of the opening 3.
Referring again to FIG. 9, an outer electrode 5, through which smoke can flow inside the sensor from the outside, is positioned axially outward of the intermediate electrode 2, with an outer chamber (outer ionization chamber) B being defined between the outer electrode 5 and the intermediate electrode 2. The inner electrode 1, the intermediate electrode 2 and the outer electrode 5 are all supported by an insulator 6 while also being insulated thereby.
With the ionization-type smoke sensor having the above-described construction, use of the doughnut-shaped intermediate electrode 2 provides the following advantages. The large opening 3 enables radiation to be efficiently projected into the outer chamber B, thereby enabling detection to be performed with increased sensitivity. Simultaneously, the electrode member 4 extending toward the center of the opening 3 enables the ionization current to be adjusted in such a manner as to prevent the characteristics of the inner chamber A from being influenced. In addition, the length of the electrode member 4 can be varied to easily vary the sensor characteristics.
In such an ionization-type smoke sensor, it is necessary that the characteristics of the ionization current within the chambers remain uninfluenced by and stable against inflow of air from the outside of the sensor.
However, in the above-described ionization type smoke sensor having the doughnut-shaped intermediate electrode, when, as shown in FIG. 9, air flows into the outer chamber B in directions towards the opening 3 of the intermediate electrode 2 from lateral or obliquely downward positions, that is, in such directions as those indicated by arrows K1', K2', K1 and K2, a phenomena such as convection occurs in the chambers, making the ionization current in the chambers unstable.